(1) Field of the Invention
The present invention relates to a method for fabricating a distributed feedback laser, and more particularly to a low distortion distributed feedback laser for analog modulation.
(2) Description of the Related Art
Conventionally, a diffraction grating of a distributed feedback laser was formed with an optical interference exposure process. With such a method, however, it was difficult to form a diffraction grating having a non-uniform distribution in pitches and depths thereof or to form a diffraction grating at partial or local regions. It was also difficult to form a diffraction grating in a large area on a semiconductor substrate.
There is a known technique for forming a laser element in which, as a method for forming non-uniform diffraction gratings, an electron beam exposure is used, and which comprises the steps of uniformly coating an electron beam resist 1 on a semiconductor substrate (InP substrate) 2 as illustrated in FIG. 1A, varying electron beam diameters during the application of the electron beam exposure, forming diffraction grating patterns whose widths are narrower at a center portion and wider towards side portions as shown in FIG. 1B, and forming diffraction gratings having different heights by a wet-etching process.
However, in the conventional method explained above in which the electron beam diameters are varied to form non-uniform grating patterns, it was difficult to control the heights of resist by the electron beam exposure, and it was also difficult to control the heights of diffraction grating by dry-etching.